Tests fail on PHP 7.4 & Drupal 9.2

Problem/Motivation

1) Drupal\Tests\migrate_example\Kernel\MigrateExampleTest::testBeerMigration
PHPUnit\Framework\Exception: PHP Fatal error:  Uncaught Exception: Serialization of 'Closure' is not allowed in Standard input code:89
Stack trace:
#0 Standard input code(89): serialize(Array)
#1 Standard input code(123): __phpunit_run_isolated_test()
#2 {main}
  thrown in Standard input code on line 89
Fatal error: Uncaught Exception: Serialization of 'Closure' is not allowed in Standard input code:89
Stack trace:
#0 Standard input code(89): serialize(Array)
#1 Standard input code(123): __phpunit_run_isolated_test()
#2 {main}
  thrown in Standard input code on line 89

See: https://www.drupal.org/pift-ci-job/2317838

Proposed resolution

Fix it.